It’s easy to tell if a given whole number is divisible by 2. Just check whether the last digit is even. There are also simple rules to determine whether a number is divisible by 3, 4, 5, 6, 8, 9, or 10. The exception is 7.
The known rules for testing for divisibility by 7 are amazingly cumbersome.
Here’s one such rule. To find out if a number is divisible by 7, double the last digit, then subtract it from the remaining digits of the number. If you get an answer divisible by 7, then the original number is divisible by 7. If you don’t know whether the new number is divisible by 7, you apply the rule again.